The present invention relates to a touch type input device.
A known touch type input device shows a pointer or the like on a display when a touch panel is touched (refer to, for example, Japanese Laid-Open Patent Publication No. 2010-9321). A user touches an operation surface defined on the touch panel of the touch type input device and, for example, selects one of a plurality of functional items shown on the display to open a desired image on the display or activate an accessory.
Such a touch panel may be of a projected capacitance type that includes a sensor pattern of first electrodes and second electrodes arranged in a grid. Touching of the touch panel is detected from the capacitance of a capacitor formed at each intersection of the first and second electrodes (refer to, for example, Japanese Laid-Open Patent Publication No. 2011-170784). In detail, in a projected capacitance type touch panel, a controller applies a cyclic drive pulse signal (voltage signal) to the sensor pattern. The controller is configured to detect a touch location where the operation surface was touched based on changes in the capacitance of the capacitors when a conductor, such as a finger, approaches or contacts the operation surface of the touch panel.
During operation of the touch panel described above, an electric field is formed outside the touch panel (touch type input device) in correspondence with the drive pulse applied to the sensor pattern. The size of the drive pulse signal cyclically changes in accordance with the frequency of the drive pulse signal. This cyclically changes the electric field formed outside the touch panel. The changes in the electric field produces radiation noise that may affect the operation of any of a various devices arranged near the touch panel.
A low-pass filter may be used to eliminate high-frequency components from the drive pulse signal and decrease changes in the electric field. In this case, however, the low-pass filter would also decrease the changes in the capacitance of the capacitors and thus lower the touching detection accuracy. Further, the frequency of the drive pulse signal may be set at a value that avoids interference with the frequency of devices arranged around the touch panel. However, when a relatively low frequency is set for the drive pulse signal, the interval at which voltage is applied to the sensor pattern becomes too long. This lowers the responsiveness of the touch panel. When a relatively high frequency is set for the drive pulse signal, the pulse width becomes too small. This lowers the detection accuracy of the touch panel.